Linear chain high molecular weight polyimides which are required for fibers and films cannot be formed from 2,2'-dicarboxybenzidine and pyromellitic dianhydride by the conventional polyamide-acid prepolymer route in dimethylacetamide because the pendant carboxyl groups participate in transamidation equilibration reactions in solution. This results in a highly solvophilic, extremely branched polyamide-acid which interferes both with spinning because of an inadequate rate of coagulation upon extrusion into a non-solvent and formation of a high molecular weight, linear polyamide-acid with pendant carboxyl groups. Pendant carboxyl groups afford (a) a mechanism for interchain hydrogen bonding and (b) enhanced solubility such that the polymer may be converted largely to imide form without losing the solubility needed for processing. The present invention avoids these problems.